1. Field of the Invention
The present invention provides an improved device for continuously measuring the electrical resistance between a grounding rod and earth ground; an alarm is triggered if the resistance is outside a preset range.
2. Prior Art
Several prior art systems are currently available that measure electrical resistance between a grounding rod and earth ground.
One such system requires the opening of the connection between the electrical safety ground and the earth grounding system. Current is injected into the earth grounding rod and a series of voltage measurements recorded. Ground resistance is calculated from these measurements. The main drawback to this approach is that the earth groundxe2x80x94safety ground link must be opened. This violates electrical code and OSHA rules unless all the equipment attached to the safety ground is turned off. Shutting down equipment to comply, such as telephone switch systems, air traffic systems and on-line computer centers is difficult to accomplish and disruptive to users of the equipment.
A second prior art system does not require that the safety groundxe2x80x94earth ground link be opened. This system utilizes the multiple electric utility safety grounds as the return path and assumes that, since there are numerous parallel earths, this return has very low resistance. Current is injected between the safety ground and the ground rod to be tested. The resulting voltage is measured, and the resistance calculated. However, the measuring device is not permanent but utilizes a hand-held clamp on meter.
What is desired is to provide a device for measuring ground current which is accurate, can be permanently affixed to a site and wherein an alarm is triggered if the resultant resistance measurement is outside a preset range.
The present invention consists of a permanently mounted AC powered control/display unit and a remote sensor for use in measuring ground resistance. The sensor is permanently mounted around the earth grounding cable. The control display unit generates a 1953 Hz, 5 Vac sine wave which is sent via a cable to a 100:1 ratio drive transformer in the remote sensor. The transformer induces a 0.05 Vac sine wave in the ground cable. The resulting current is detected by a 100:1 turns ratio sense transformer. The current is returned via the cable to the control display unit and converted to a voltage, filtered, amplified and rectified by a synchronous rectifier. The rectified voltage is again filtered and presented to an analog to digital converter. A microprocessor reads the output of the analog to digital converter and the ground resistance is computed by using Ohm""s Law (R=E/I), the result being shown on a display.
The synchronous rectifier is an analog switch which is alternately connected to the sense voltage and to ground. The switch is controlled by the same 1953 Hz square wave that also, after filtering, drives the drive transformer. A synchronous rectifier has two advantages. In particular, any noise at frequencies other than the control frequency (1953 Hz) will average to zero. In addition, a 1953 Hz sine wave that is 90 degrees out of phase with the control square wave will also average to zero.
Using a synchronous rectifier in the ground measuring device of the present invention reduces the complexity of the remote sensor by removing the need for precision magnetic cores, complex distributed coil winding, and extensive magnetic shielding to prevent interaction between the drive and sense transformers. Placing a calibration table in nonvolatile memory in each remote sensor greatly reduces the cost and complexity of the remote sensor since standard tolerance parts and processes can be used and still maintain the interchangeability and accuracy requirements.
The present invention thus provides a ground resistance sensing device which is more accurate and less expensive than those devices currently available in the prior art.